Expert: Walter Hintz - 5/19/2007

Question
HI,
Im in year 8 and i have been set homewok kthat i cant do.
The question is...
Devise experiments to show that the air we breathe out contains more carbon dioxide and less oxygen than the air we breate in.
I think the experiment is the lime water going milky, but i dont know how how to show that theres more carbon dioxide.

Plese dont make the answer too complex!
ellie

Answer
Hi Ellie
 You are correct that limewater can be used.
Limewater is calcium hydroxide Ca(OH)2.  It is normally clear. If you pour limewater into a container it mixes with the air which is 21 % oxygen and only .038% CO2.  As you probably know air is mostly nitrogen. So here is what happens
  When you inhale 21% O2  Less than 1 % CO2
  When you exhale 15% O2 and 4.5% CO2

 How does the limewater work?  
 When limewater is mixed with outside air it remains clear
 If you breath through a straw into limewater you are putting 4.5% CO2 into the lime water and here is what happens
 The calcium in the calcium hydroxide combines with the CO2 and forms calcium carbonate which appears as a milky solid.
 Hope this helps